A simple satellite does not normally follow a free-fall gravitational orbit, but is subject to drag forces due to the residual atmosphere, radiation pressure, collisions with meteroids, etc. This requires frequency updating of the satellite ephemeris in order to use it for navigational purposes.
A new concept called a zero-drag or drag-free satellite incorporates very small jets and a fuel supply to just counteract the drag forces. This is accomplished by incorporating into the satellite a proof-mass which is shielded from all effects except gravity. The position of the proof-mass relative to the rest of the satellite is sensed by a pick-off and the jets are controlled to keep the pick-off at null and thus make the satellite follow the proof-mass which is in free-fall since the null position of the proof-mass is made to be at a position where the gravitational effects of the rest of the satellite cancel.
An electronic pick-off is the only pick-off embodiment reduced to practice, and is very critical due to the large gaps required. This leads to the alternative, such as the optical shadowing apparatus of the present invention which provides highly desirable results.